That's it. When evaluating a plus
expression, the plus_eval
callable gets called with the left hand side and right hand side expression
and the associated Context.

But there is more: As Actions can
be full fletched Proto
Transforms, you can in fact use any proto expression you can imagine
as the action. Phoenix predifines a set of callables and transform to deal
with the Context information passed along and of course every Phoenix expression
can be used as a Phoenix grammar or Proto
Pass Through Transform.